Press Release

Cosmic Archeology Uncovers the Universe’s Dark Ages

September 13, 2006

Astronomers using the Subaru telescope in Hawai'i have looked 60 million
years further back in time than any other astronomers, to find the most
distant known galaxy in the universe. In doing so, they are upholding
Subaru's record for finding the most distant and earliest galaxies known.
Their most recent discovery is of a galaxy called I0K-1 that lies so far
away that astronomers are seeing it as it appeared 12.88 billion years ago.

This discovery, based on observations made by Masanori Iye of the National
Astronomical Observatory of Japan (NAOJ), Kazuaki Ota of the University of
Tokyo, Nobunari Kashikawa of NAOJ, and others indicates that galaxies
existed only 780 million years after the universe came into existence about
13.66 billion years ago as a hot soup of elementary particles.

To detect the light from this galaxy, the astronomers used Subaru
telescope's Suprime-Cam camera outfitted with a special filter to look for
candidate distant galaxies. They found 41,533 objects, and from those
identified two candidate galaxies for further study using the Faint Object
Camera and Spectrograph (FOCAS) on Subaru. They found that IOK-1, the
brighter of the two, has a redshift of 6.964, confirming its 12.88
billion-light-year distance.

The discovery challenges astronomers to determine exactly what happened
between 780 and 840 million years after the Big Bang. IOK-1 is one of only
two galaxies in the new study that could belong to this distant epoch.
Given the number of galaxies that have been discovered from 840 million
years after the Big Bang, the research team had expected to find as many
as six galaxies at this distance. The comparative rarity of objects like
IOK-1 means that the universe must have changed over the 60 million years
that separate the two epochs.

The most exciting interpretation of what happened is that we are seeing an
event known to astronomers as the reionization of the universe. In this
case, 780 million years after the Big Bang, the universe still had enough
neutral hydrogen to block our view of young galaxies by absorbing the light
produced by their hot young stars. Sixty million years later, there were
enough hot young stars to ionize the remaining neutral hydrogen, making the
universe transparent and allowing us to see their stars.

Another interpretation of the results says that there were fewer big and
bright young galaxies 780 million years after the Big Bang than 60 million
years later. In this case, most of the reionization would have taken place
earlier than 12.88 billion years ago.

No matter which interpretation finally prevails, the discovery signals that
astronomers are now excavating light from the "Dark Ages" of the universe.
This is the epoch when the first generations of stars and galaxies came
into existence, and an epoch which astronomers have not been able to
observe until now.

BACKGROUND INFORMATION:

Archeology of the Early Universe Using Special Filters
Newborn galaxies contain stars with a wide range of masses. Heavier stars
have higher temperatures, and emit ultraviolet radiation that heats and
ionizes nearby gas. As the gas cools it radiates away excess energy so that
it can return to a neutral state. In this process, hydrogen will always
emit light at 121.6 nanometers, called the Lyman-alpha line. Any galaxy
with many hot stars should shine brightly at this wavelength. If stars
form all at once, the brightest stars could produce Lyman-alpha emission
for 10 to 100 million years.

In order to study galaxies like IOK-1 that exist at early times in the
universe, astronomers must search out Lyman-alpha light that is stretched
and redshifted to longer wavelengths as the universe expanded. However, at
wavelengths longer than 700 nanometers, astronomers have to deal with
foreground emissions from OH molecules in Earth's own atmosphere that
interfere with faint emissions from distant objects.

To detect the faint light from distant galaxies, the research team had been
observing at wavelengths where Earth's atmosphere doesn't glow much,
through windows at 711, 816, and 921 nanometers. These windows correspond
to the redshifted Lyman-alpha emission from galaxies with redshifts of
4.8, 5.7, and 6.6, respectively. These numbers indicate how much smaller
the universe was compared to now, and correspond to 1.26 billion years,
1.01 billion years, and 840 million years after the Big Bang. This is like
doing archaeology of the early universe with particular filters allowing
scientists to see into different layers of an excavation.

To obtain their spectacular new results, the team had to develop a filter
sensitive to light with wavelengths only around 973 nanometers, which
corresponds to Lyman alpha emission at a redshift of 7.0. This wavelength
is at the limit of modern CCDs, which lose sensitivity at wavelengths
longer than 1000 nanometers. This one of its kind filter, called the
NB973, uses multilayer coating technology, and took more than two years to
develop. Not only did the filter have to pass light with wavelengths only
around 973 nanometers, but it also had to cover uniformly the entire field
of view of the telescope's prime focus. The team worked with a company,
Asahi Spectra Co.Ltd, to design a prototype filter to use with Subaru's
Faint Object Camera, and then applied that experience to making the filter
for Suprime-Cam.

The Observations
The observations with the NB973 filter took place during the spring of
2005. After more than 15 hours of exposure time, the data obtained
reached a limiting magnitude of 24.9. There were 41,533 objects in this
image, but a comparison with images taken at other wavelengths showed that
only two of the objects were bright only in the NB973 image. The team
concluded that only those two objects could be galaxies at a redshift of
7.0. The next step was to confirm the identity of the two objects, IOK-1
and IOK-2, and the team observed them with the Faint Object Camera and
Spectrograph (FOCAS) on the Subaru telescope. After 8.5 hours of exposure
time, the team was able to obtain a spectrum of an emission line from the
brighter of the two objects, IOK-1. Its spectrum showed an asymmetrical
profile that is characteristic of Lyman-alpha emission from a distant
galaxy. The emission line was centered at a wavelength of 968.2 nanometers
(redshift 6.964), corresponding to a distance of 12.88 billion light years
and time of 780 million years after the Big Bang.

The Identity of the Second Candidate Galaxy
Three hours of observation time did not yield any conclusive results to
determine the nature of IOK-2. The research team has since obtained more
data that is now being analyzed. It is possible that IOK-2 may be another
distant galaxy, or it could be an object with variable brightness. For
example, a galaxy with a supernova or a black hole actively swallowing
material that just happened to appear bright during the observations with
the NB973 filter. (Observations in the other filters were made one to two
years earlier.)

The Subaru Deep Field
The Subaru telescope is particularly well suited for the search of the most
distant galaxies. Of all the 8- to 10-meter-class telescopes in the world,
it is the only one with the ability to mount a camera at prime focus. The
prime focus, at the top of the telescope tube, has the advantage of a wide
field of view. As a result, Subaru currently dominates the list of the most
distant known galaxies. Many of these are in a region of the sky in the
direction of the constellation Coma Berenices called the Subaru Deep Field that the research team selected for intense study at many wavelengths.

The Early History of the Universe and the Formation of the First Galaxies
To put this Subaru accomplishment into context, it is important to review
what we know about the history of the early universe. The universe began
with the Big Bang, which occurred about 13.66 billion years ago in a fiery
chaos of extreme temperature and pressure. Within its first three minutes,
the infant universe rapidly expanded and cooled, producing the nuclei of
light elements such as hydrogen and helium but very few nuclei of heavier
elements. In 380,000 years, things had cooled to a temperature of around
3,000 degrees. At that point, electrons and protons could combine to form
neutral hydrogen.

With electrons now bound to atomic nuclei, light could travel through space
without being scattered by electrons. We can actually detect the light that
permeated the universe back then. However, due to time and distance, it has
been stretched by a factor of 1,000, filling the universe with radiation we
detect as microwaves (called the Cosmic Microwave Background). The
Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft studied this
radiation and its data allowed astronomers to calculate the age of the
universe at about 13.66 billion years. In addition, these data imply the
existence of such things as dark matter and the even more enigmatic dark
energy.

Astronomers think that over the first few hundred million years after the
Big Bang, the universe continued to cool and that the first generation of
stars and galaxies formed in the densest regions of matter and dark matter.
This period is known as the "Dark Ages" of the universe. There are no
direct observations of these events yet, so astronomers are using computer
simulations to tie together theoretical predictions and existing
observational evidence to understand the formation of the first stars and
galaxies.

Once bright stars are born, their ultraviolet radiation can ionize nearby
hydrogen atoms by splitting them back into separate electrons and protons.
At some point, there were enough bright stars to ionize almost all the
neutral hydrogen in the universe. This process is called the reionization
of the universe. The epoch of reionization signals the end of the Dark
Ages of the universe. Today most of the hydrogen in the space between
galaxies is ionized.

Pinpointing the Epoch of Reionization
Astronomers have estimated that reionization occurred sometime between 290
to 910 million years after the birth of the universe. Pinpointing the
beginning and end of the epoch of reionization is one of the important
stepping stones to understanding how the universe evolves, and is an area
of intense study in cosmology and astrophysics.

It appears that as we look farther back in time, galaxies get rarer and
rarer. The number of galaxies with a redshift of 7.0 (which corresponds to
a time about 780 million years after the Big Bang) seems smaller than what
astronomers see at a redshift of 6.6 (which corresponds to a time about 840
million years after the Big Bang). Since the number of known galaxies at a
redshift of 7.0 is still small (only one!) it is difficult to make robust
statistical comparisons. However, it is possible that the decrease in
number of galaxies at higher redshift is due to the presence of neutral
hydrogen absorbing the Lyman-alpha emission from galaxies at higher
redshift. If further research can confirm that the number density of
similar galaxies decreases between a redshift of 6.6 and 7.0, it could mean that IOK-1 existed during the epoch of the universe's reionization.

These results will be published in the September 14, 2006, edition of Nature.

Table 1: The 10 most distant known galaxies as of September 14, 2006. This
list contains objects whose redshifts have been confirmed by spectroscopy.
Other potentially high redshift galaxies have been discovered, but their
identity has not been confirmed spectroscopically. The redshift is a
measure of how much the universe has expanded since an object emitted its
light. A redshift of 7 would mean that an object existed when the universe
was 8 (redshift plus one) times smaller than its current size. The
distances are derived from the redshifts using the same cosmological model
as previous press releases from Subaru Telescope (H0=71km/s/Mpc,
Ω=0.27, Λ=0.73). Using a different cosmological model will
result in different distances even for the same redshift. Since there are
different models of the universe that are consistent with observations,
the age of the universe has an ambiguity of a few 100 million years.

Figure 1: Snapshots of the universe at different epochs. From the top right
corner, fluctuations in the density of matter 380 thousand years after the
Big Bang (from NASA's WMAP satellite's data on the Cosmic Microwave
Background Radiation; http://map.gsfc.nasa.gov/), the growth in density
variations several hundred million years later (from the Virgo Consortium; http://www.virgo.dur.ac.uk/new/index.php), the new Subaru observations from
780 million years after the Big Bang, earlier Subaru observations from 840
million years and 1.01 billion years after the Big Bang, and the present. (Enlarge)

Figure 2: A series of images zooming in on Galaxy IOK-1, the reddish object
in the center of the last panel, currently the most distant known galaxy
about 12.88 billion light years away. The wide field image is a 254 by 284
arcsecond (North is up, East is left) portion of the entire region observed
in search for distant galaxies. The closeup image is 8 by 8 arcseconds. (Enlarge)

Figure 3: OH emission lines of Earth's atmosphere and the windows used with
the Subaru telescope for observing Lyman alpha emission from distant
galaxies. Windows exist at 711, 816,921, and 973 nanometers. The
horizontal axis shows the wavelengths, the vertical axis shows the
relative brightness of Earth's atmosphere at those wavelengths. (Enlarge)

Figure 4: The NB973 Filter compared to the size of a Japanese 10 yen coin,
about 2.5 cm (one inch) in diameter. It looks black because it passes no
light at wavelengths that the human eye is sensitive to. Masanori Iye of
the National Astronomical Observatory of Japan, in collaboration with
Asahi
Spectra, developed the filter with funding from Japan's Ministry of
Education, Culture, Sports, Science and Technology. (Enlarge)

Figure 5: Images of IOK-1 and IOK-2, the two candidates for record breaking
distant galaxies, at various wavelengths. They are only visible in the
image taken with the NB973 filter, a necessary requirement for a galaxy
with a redshift around 7. (Enlarge)

Figure 6: The spectrum of IOK-1. It shows Lyman alpha emission at a
redshift of 6.964. The top panel is the spectrum as detected by CCD. The
middle panel shows spectra as a graph with wavelength on the horizontal
axis and the brightness of emission on the vertical axis. The red line is
the spectrum of IOK-1. The blue line shows the typical profile of Lyman
alpha emission from galaxies at a redshift of 6.6 shifted in wavelength to
line up with IOK-1's spectrum. IOK-1's spectrum shows the characteristic
profile of a Lyman alpha line from a distant galaxy with a steeper slope
at shorter wavelengths. The bottom panel shows the strength of Earth's
atmosphere's emission at these wavelengths. (Enlarge)

Figure 7: The change of the number density of galaxies (top panel) and star
formation rate (bottom panel) with a change in redshift. The small black
dot represents the results if both of the two candidate galaxies are at a
redshift of 7.0. The big black dot is the result if only IOK-1 is at a
redshift of 7.0. The error bars represent uncertainties due to small number
statistics and the variations in density across the universe. In either
case, the number density seems to be decreasing at a redshift of 7.0. If
this is confirmed, it may imply that the reionization of the universe was
not completed by a redshift of 7.0. (Enlarge)

Note: Throughout this article redshifts were converted to distances and
ages using cosmological parameters of H0=71km/s/Mpc, Ω=0.27, and
Λ=0.73 to maintain consistency with previous releases. The research
paper submitted to Nature reports slightly different numbers based on
cosmological parameters of H0=70km/s/Mpc, Ω=0.3, and Λ=0.7.